Ivygen Error when I add ivy
Old question. I'm using Blender 2.79, but I had the same issue. I also didn't realize IvyGen comes pre-installed with blender; and after installing it I got this issue.

Blender doesn't overwrite plugins (at least this version - I can't vouch for 2.77, but I'd doubt it). Instead, it stores plugins in the AppData directory. I got this error, which clarifies:



After deleting the copy of the plugin from my AppData, then restarting Blender, I'm able to enable the correct copy of the plugin, and it now runs fine.

Note that, adding an ivy curve to a Blender default startup file with a cube may not appear to work immediately. This happened to me because my cursor was inside the cube, since the ivy (as when any other objects are added via Shift+A) grows from the 3D cursor's location.